Non-alcoholic fatty liver disease (NAFLD) is one of the major health concerns worldwide. However, the underlying mechanism has been poorly understood. MicroRNAs (miRNAs) are small (19-22 nucleotide) noncoding RNA molecules that bind to the 3' untranslated region (3'-UTR) of mRNAs to post- transcriptionally regulate gene expression, usually leading to inhibition of gene expression. Recent data have demonstrated that miRNAs play an important role in regulating lipid and glucose metabolism as well as many other biological processes. Nuclear hormone receptors are ligand-activated transcription factors that regulate both development and adult physiology. Hepatocyte nuclear factor 4? (HNF4?) is a member of the nuclear hormone receptor superfamily and has been shown to play a critical role in maintaining bile acid, lipid and glucose homeostasis. Although loss of hepatic HNF4? is known to cause liver steatosis, the role of HNF4? in the progression of NAFLD has not been investigated before. Our preliminary data have shown that 1) patients with non-alcoholic steatohepatitis (NASH) have a significant increase in miR-34a and miR-149 expression and a marked decrease in HNF4? expression, 2) diabetic mice or high fat diet (HFD)-fed mice also have increased hepatic miR-34a and miR-149 expression and a significant reduction in hepatic HNF4? expression, and 3) over-expression of hepatic miR-34a or miR-149 causes a reduction in hepatic HNF4? expression and an increase in hepatic triglyceride (TG) levels. Based on these preliminary data, we hypothesize that in response to specific metabolic cue(s), a cascade involving miR-34a, miR-149 and HNF4? is activated to increase hepatic TG levels, therefore facilitating the development and progression of NAFLD. To test this hypothesis, we will utilize both gain- and loss-of-function approaches to further characterize the role of miR-34a, miR-149 and HNF4??in hepatic TG metabolism and the development and progression of NAFLD. Accomplishing the specific aims proposed in this application will provide novel and important insights into the mechanism underlying the pathogenesis of NAFLD, and may lead to identification of novel target(s) for treatment of NAFLD.